1. Field of the Invention
The present invention relates to a color imaging element, and, in particular, relates to a color imaging element which can reduce generation of color moire and convert a resolution into a higher resolution.
2. Description of the Related Art
In a single-plate color imaging element, a color filter of a single color is provided on each pixel, and therefore each pixel has only color information of a single color. Hence, an output image of a single-plate color imaging element becomes a RAW image (mosaic image), and therefore multichannel images are obtained by processing (demosaicing processing) of interpolating pixels of deficient colors from surrounding pixels. What matters in this case is reproduction characteristics of a high frequency image signal. Since the color imaging element is likely to cause aliasing in a captured image compared to a monochrome imaging element, an important task is to convert a resolution into a high resolution by expanding a reproduction band while suppressing generation of color moire (false color).
The demosaicing processing refers to processing of calculating all pieces of color information per pixel from a mosaic image corresponding to a color filter array of a single-plate color imaging element, and is also referred to as concurrent processing. For example, when an imaging element includes color filters of three colors RGB, the demosaicing processing is the process for calculating all pieces of color information of RGB per pixel from a mosaic image configured by RGB.
In a primary color Bayer array, which is the most widely used color array of color filters in the single-plate color imaging elements, green (G) pixels are arranged in a checkered pattern and red (R) and blue (B) are arranged in a line sequence, and therefore there is a problem with reproduction precision when G signals generate high frequency signals in diagonal directions, and when R and B signals generate high frequency signals in horizontal and vertical directions.
When a monochrome vertical stripe pattern (high frequency image) as indicated by the A portion in FIG. 25 is incident on a color imaging element including color filters of a Bayer array indicated by the B portion in FIG. 25, a color image of a mosaic pattern is provided in which R is light and flat, B is dark and flat and G is a light and dark as indicated by the C portion to E portion in FIG. 25 upon comparison with each color by sorting the pattern into the Bayer color array. Originally, a density difference (level difference) is not produced among RGB since the image is monochrome, but depending on a color array and an input frequency, a color is applied to the image.
Similarly, when a diagonally monochrome high frequency image as indicated by the A portion in FIG. 26 is incident on an imaging element including color filters of a Bayer array indicated by the B portion in FIG. 26, a color image is provided in which R and B are light and flat and G is dark and flat as indicated by the C to E portions in FIG. 26 upon comparison with each color by sorting the pattern into the Bayer color array. If a value of black is 0 and a value of white is 255, the diagonally monochrome high frequency image becomes green-colored since only G takes 255. Thus, the Bayer array cannot correctly reproduce a diagonal high frequency image.
Generally, in an imaging apparatus which uses single-plate color imaging elements, optical low pass filters made of a birefringent material such as crystal are arranged in front of the color imaging elements to optically suppress a high frequency wave. This method can reduce a tinge due to aliasing of a high frequency signal, but has a problem that the resolution lowers due to a negative effect of this method.
To solve such a problem, color imaging elements is proposed which adopt a three color random array which satisfies array limitation conditions that arbitrary pixels of interest are adjacent to three colors including colors of the pixels of interest in one of four sides of the pixels of interest (Japanese Patent Application Laid-Open No. 2000-308080; PTL 1).
Further, an image sensor is proposed which has a plurality of filters of different spectral sensitivities having a color filter array in which first filters and second filters are alternately arranged in a first predetermined cycle in one of diagonal directions of a pixel grid of the image sensor, while they are alternately arranged in a second predetermined cycle in the other one of the diagonal directions (Japanese Patent Application Laid-Open No. 2005-136766; PTL 2).
Furthermore, in a color solid state imaging element of three primary colors of RGB, a color array is proposed which makes each appearance probability of RGB equal, and allows arbitrary lines (horizontal, vertical and diagonal lines) on an imaging plane to transit all colors by arranging sets of three pixels of horizontally-arranged R, G and B in a zig-zag pattern in the vertical direction (Japanese Patent Application Laid-Open No. 11-285012; PTL 3).
Still further, a color imaging element is proposed in which R and B of the three primary colors of RGB are arranged every three pixels in the horizontal and the vertical directions, and G is arranged between these R and B (Japanese Patent Application Laid-Open No. 8-23543; PTL 4).